From a git perspective, you would:
- Clone the Anvil app to your local machine (or pull from Anvil to update it)
- Make changes to the repo on your local machine (adding files, etc)
- Commit and push those changes back to Anvil
Depending on what you’re doing in #2, you may or may not have any luck. I’m sure there are things you could do that would break the Anvil app, just as there are things you could do that would not get you what you wanted. But at least you could always revert to the previous commit and push to Anvil to get out of trouble.
If you’re talking about embedding another repository inside the Anvil app’s repository, I’m sure there are ways of doing that in git as well. They’d be done in the #2 step above. That’s beyond my git knowledge, though.